1. Field of the Invention
This invention relates to a light diffusing film and its manufacture, and a polarizing plate with a light diffusing layer, and a liquid crystal display apparatus which are suitably used for the display of a word-processor, computer, television set or the like, for a polarizing plate or an optical lens to be incorporated in a liquid crystal display apparatus, and as for a cover of various meters, and as the window-shield glass of an automobile or of a railway coach.
2. Prior Art
The display apparatus of a computer or the like as described above uses various light diffusing films to improve its visibility.
Of such light diffusing films some depend on surface roughness to assume a light diffusing activity while others depend on a diffusing material contained in a resin film where the difference in refractive indices between the resin and the diffusing material is responsible for the light diffusing activity.
When the light diffusing activity of a given film depends on its surface roughness, the light diffusing activity and the transparency of film vary according to from which angle it is seen, and thus the visibility thereof varies according to from which angle it is seen, which poses a problem. For example, with a display apparatus incorporating such a film, an image on display is quite visible when seen from front, but the same image becomes whitish when the visual angle becomes large.
Such a light diffusing film containing a diffusing material in its resin substrate as described above is disclosed, to take as an example a film applied to a reflection type liquid crystal display apparatus, in Journal of Illumination Research Society MD-96-48 (1996) pp. 277-282.
According to this disclosure, to obtain a reflection type liquid crystal display apparatus with a high contrast, it is important to design the apparatus such that the frontal light diffusing intensity is appropriately adjusted, and the backward light diffusing intensity is strongly suppressed, because such design is advantageous for the realization of a bright display and a high resolution, and further ensures the realization of a birefringent film which will compensate for the optical performance of liquid crystal which, in turn, will be beneficial for widening the effective view angle.
The disclosure further asserts that, to achieve above object, it will be better to use a film where light diffusing occurs as a result of mismatch in refractive indices among film constituents, rather than a film where light diffusing occurs as a result of surface roughness, because with the latter film the backward light diffusing intensity will become so large as to lower the contrast. Namely, the disclosure recommends the use of a resin film which contains a diffusing material within, where light diffusing activity is invoked by the difference in refractive index among film constituents.
The above-cited article in Journal of Illumination Research Society MD-96-48 states, utilizing the light diffusing theory offered by Mie and the theory by Hartel as a concept to support the design of an effective light diffusing film, it is possible to obtain a film with an optimum light diffusing activity, by appropriately adjusting the relative refractive index m between a diffusing material and a resin, and the size parameter .alpha. and particle density parameter Nd of diffusing material, and particularly it is useful for achievement of above purpose to keep the size parameter .alpha. at 10 or more.
The size parameter .alpha. is .alpha.=2.pi.R/.lambda., and thus it depends on the diameter R of diffusing material (.lambda. represents the wavelength of light).
Accordingly, if the size parameter .alpha. be 10 or more, inevitably the diffusing material will have a large particle size in association. If a diffusing material with such a large particle diameter consisting, for example, of spherical particles made of a plastics, is allowed to disperse in a transparent polymer (resin), the resultant film will become thick, and thus usable combinations of a diffusing material and resin for molding will be limited, or the film will develop a birefringence when molded by extrusion. These pose a problem when the film is put to practice.
Further, when such a light diffusing film is applied to a liquid crystal display, it must be applied to the outside surface of liquid crystal panel, because, if it were applied to the inside surface of liquid crystal panel, it would interfere with the polarizing activity of the display unit.
The alternative, conventional method whereby reflection of rays incident on the surface of a transparent substrate is prevented includes a method wherein an anti-reflection coat is applied on the surface of a glass or plastics substrate, a method wherein an ultra-thin film made of MgF.sub.2 or other metals having a thickness of about 0.1 .mu.m is applied by vapor deposition on a transparent substrate made of glass or the like, a method wherein an ionizing-radiation setting resin is coated on the surface of a plastics acting as a plastics lens, and then another film made of SiO.sub.2 or MgF.sub.2 is plated by vapor deposition on the former, and a method wherein an ionizing-radiation setting resin is coated and on that hardened coat is applied a film with a low refractive index.
However, for those conventional films to be given a light diffusing activity, only the outermost layer is available, and thus they can not have an anti-reflection layer if a light diffusing layer is prepared on the outermost layer. Hence, if such a light diffusing film is applied onto the surface of panel of a liquid crystal display apparatus, reflection of rays incident on the surface is not sufficiently prevented.
Further, with a conventional reflex type liquid crystal apparatus, it is generally a custom to place polarizing plates on both surfaces of a liquid crystal cell, and to add to one of them a diffuse reflection plate having a roughened surface.
However, when the diffuse reflection plate is placed on the outside surface of liquid crystal cell, a ghost display called parallax develops depending on the thickness of glass of liquid crystal cell, which greatly impairs the visibility of the display.
To prevent the development of parallax, a method has been developed wherein only a single polarizing plate is used, and a metal electrode in the liquid crystal cell is allowed to act also as a light reflection agent. When the metal electrode assumes a light reflecting property by acting as a mirror, the visual angle becomes narrow, and brightness along the normal decreases. When the metal electrode assumes a light diffuse reflection property by having a roughened surface, it becomes difficult to control the orientation of liquid crystal, and the production processes become complicated. This poses a new problem.